Anode and grid bias voltage system for class b or c amplifiers



Oct. 24, 1950 P. A. DONKER 2,527,406- ANODE AND. GRID BIAS VOLTAGE SYSTEM FOR cLAss B 0R c AMPLIFIERS Filed May 9, 1946 PJFZERADRLAANDOAKER IN V EN TOR.

Arrow Patented Oct. 24, 1950 ANODELAND: GRID BIAS VOLTAGE SYSTEM FOR CLASS B OR C AMPLIFIERS Pieter Adriaan Donker, Eindhoven, Netherlands, assignor, by mesne assignments, to Hartford National Bank and Trust Company, Hartford,

@onn as-trustee Applieationli/lay 9, 1946, Serial No. 668,424 In the Netherlands July 4, 1941 Section 1, PublicLaw csamAugust s, 1946 Patent expires July 4, 1961 Claims. 1

The invention relates to a push pull amplifying system wherein such a negative bias voltage is supplied to the control grids of the amplifying tubes that in uncontrolled condition the anode current of the amplifying tubes is substantially suppressed (B-class amplifier) or wherein use is made of a larger grid bias voltage-than is required for the suppression of the anode current (C-class amplifier).

With such amplifiers wherein the anode voltage is taken from an anode voltage apparatus provided with a supply transformer, it is known to reduce or to prevent distortion of the amplified oscillations which is caused by the-internal resistance of the anode voltage apparatus, by supplying to the control grids of the amplifying tubes not only a fixed negative bias voltage but also a variable bias voltage which counter-acts the negative bias voltage and which depends on the load on the amplifier.

With known push-pull amplifying systems the required grid bias voltage which increases with the control of the amplifier is obtained by connecting an impedance into the output circuit of the amplifier, by rectifying that portion of the output energy which is taken therefrom and by supplying the load-dependent direct-current voltage thus obtained, after being smoothed, to the control grids.

It is also known to derive the variable grid bias voltage from the anode direct-current voltage itself, in which event the grid bias voltage is o'ctained by rectifying and smoothing the oscillations of an auxiliary oscillator whose anode voltage is formed by that of the amplifying tubes.

Although the last-mentioned method of generating the variable grid-bias voltage has the advantage that now the energy required therefor is taken, in contradistinction to the first-mentioned method, directly from the anode voltage apparatus, the last-mentioned method has the drawback of being comparatively complicated and expensive.

The invention aims at avoiding the abovementioned drawbacks.

According to the invention, the variable gridbias voltage is obtained by rectifying and smoothing. an alternating current or voltage which occurs in the anode-voltage apparatus and which depends on the control of the amplifier.

The variable grid-bias. voltage is preferably taken from the secondary winding of a transformer whose primary winding is connected in series with the anode'woltage winding of the supply transformer whilst, in order to reduce the voltage drop thus produced in the anode voltage drop produced across rectifier circuit, the ratio of transformation of the transformer is so chosen, that the voltage the primary windings thereof is smallwith respect to the output voltage.

The invention will be explained more fully with reference. to the accompanying drawing which represents, byway of example, one particularly advantageous embodiment of a push-pull amplifying; system: according to the invention.

In the amplifying system shown the, for example, low-frequency oscillations to be amplified are supplied to the input terminals l of a preamplifier 2 whose output circuit comprises a transformer: 5. This transformer has a secondary winding e which is divided, into two equal parts which are connected into the control-grid circuits of two amplifying tubes 5 and 6 respectively which are connected in push-pull and as B- class. With the aid of an output transformer 1 the amplified oscillations are supplied to the load impedance formed, for example, by a loudspeaker 8.

The anode voltages of the amplifying tubes 5 and 6 are taken from an anode voltage apparatus which com-prises a supply transformer 51 which is to be connected to an alternating current supply, a full-wave rectifying tube it and a smoothing filter-consisting of condensers l l and a choke coil l2. The negative and. positive output terminals I3 and [4' respectively of the anode-voltage apparatus are connected to the earthed cathodes of the amplifying tubes 5 and 6 and to the central tap on the primary winding of the output transformer 1 respectively.

The variable negative gridbias voltage required for the tubes 5, B connected as a B-class rectifier is taken from the alternating current supply with the aid of an additional supply transformer 15 and after being rectified by a full-wave rectifying tube t6 and smoothed by means of a filter l1, it is supplied via the halves of the secondary winding of the input transformer 3 to the control grids of the amplifying tubes.

. In order to avoid distortion of the amplified oscillations clue to an increase of the internal voltage drop of the anode-voltage apparatus and therefore a decrease of the anode voitage upon an increase of the control of the amplifying tubes 5, 5, the control grids: have supplied to them not only the fixed negative bias voltage taken from the transformer IE but also a variable bias voltage which counteracts the said fixed negative bias and which increases with the load on the anode voltage apparatus and therefore with the control of the push-pull amplifier.

According to the invention, this. variable bias voltage is derived from an alternating current or voltage which occurs in the anode-voltage apparatus and which increases with the load on this apparatus. For example, the variable grid voltage may be taken from the choke coil by providing a secondary winding on the core of the said coil and by supplying the alternating voltage occurring across the said winding, after being rectified and smoothed, to the control grids of the tubes.

A further possibility of realising the system according to the invention, which has proved to be particularly advantageous, is represented in the drawing. s

In this form of construction the variable bias voltage is taken from the secondary winding of a transformer I8 whose primary winding I8 is connected between the two halves 20 and 2I in which the secondary winding of the supply transformer 9 is divided in view of the full-wave rectifier I whilst the central tap on the primary winding I9 forms the negative output terminal of the anode-voltage apparatus. Through the primary winding I9 passes consequently the anode current of the rectifying tube III, which current increases with the increasing load on the anodevoltage apparatus. As a result there is induced in the secondary winding 22 of the transformer I8, whose secondary winding-halves are oppositely connected in series with the secondary winding-halves 23, 24 of the transformer I5, an alternating voltage whose amplitude increases with the load on the anode-voltage apparatus.

Since the secondary windings of the transformers I5 and I8 are oppositely connected in series and therefore the output voltages of both transformers counteract one another the resulting alternating voltage set up across the rectifying tube I6 has a maximum instantaneous value which decreases with an increasing load on the anode-voltage apparatus with the result that with an increasing load on the anode-voltage apparatus the negative grid bias voltage supplied to the first smoothing condenser 25 of the filter I! and consequently to the control grids of the tubes 5 and 6 decreases owing to the increasing control of the push-pull amplifier, which result was aimed at. In order to be able to adjust the grid-bias voltage in each of the tubes 5 and 6 exactly to the desired value, the output voltage of the filter I1 is supplied, via potentiometers 28 and 29 respectively and after being further smoothed with the aid of condenser 30 and 3I respectively, to the control grid concerned.

In order to prevent an unnecessary increase of the internal voltage drop of the anode-voltage apparatus with an increasing load, the voltagev drop occurring in the primary winding I9 is preferably reduced as far as possible. In view thereof it is advantageous to take a high ratio of transflormation of the transformer I8, for example of Besides, in order to restrict the voltage drop in the primary winding of the transformer I8, it is advantageous to ensure that the output voltages of the transformers I5 and I8 are exactly in counterphase so that the influence exerted by the alternating voltage taken from the transformer I8 on the grid voltages of the tubes is as great as possible.

Since, as is known in itself, the current in the anode current rectifier circuit is not exactly in phase with the voltage across the secondary winding of the transformer 9, the output voltages of the transformers I5 and I8 are neither exactly in counterphase without particular steps are taken. In order to ensure in the circuit arrangements shown that the said output voltages are exactly in counterphase, a condenser 26 connected in series with a resistance 2! is connected in parallel to each of the secondary windinghalves 23, 24 and a voltage which is exactly in counterphase with the output voltage of the transformer I8 and which is out of phase with respect to the alternating voltage across the winding-halves 23, 24 is taken from the resistance 21.

Care should further be taken to prevent the load on the grid voltage rectifier circuit from becoming excessively high, so that the potentiometer resistances 28, 29 should be taken of comparatively high values, taking therewith into account that the time constant for the charge and the discharge of the condensers 25, 30, 3| should amount to about 0.1 second in order to be able to follow rapid variations in the load on the amplifier.

It may be observed that in view of distortion caused by the grid current of the tubes 5, 6, it is also advantageous to take comparatively large resistances 28 and 29, in which event, upon the occurrence of a slight grid current, the condensers 30 and 3| are charged to a voltage which approximately corresponds to the maximum instantaneous value of the grid-control voltage whilst the energy required for charging the condensers and which is taken from the final or energy amplifying stage of the pre-amplifier 2 is only in this case.

A supplementary advantage of the above-mentioned increase of the negative grid-bias voltage upon the occurrence of grid current is that thus the production of an inadmissibly high anode current in the tubes 5 and 6 is counteracted.

In order to prevent the tubes 5 and 6 from being overloaded, it has furthermore proved to be advantageous to dimension the transformer I8 from which is taken that part of the total gridbias voltage which varies with the load, in such manner that with the highest admissible load on the amplifying tubes 5 and 6 the iron of the core of the transformer I8 just gets saturated. Upon an increase of the load above the highest admissible load on the amplifying tubes 5 and 6 the absolute value of the grid-bias voltage no longer decreases with the result that a further increase of the anode current of the tubes 5 and 6 is not supported.

In the shown practical example of the system according to the invention the tubes 5 and 6 are represented as triodes. The invention, may, however, also be applied, of course, to tubes having more than three electrodes and the effect aimed at may also be obtained by supplying the variable grid-bias voltage to auxiliary grids instead of to the control grids.

It may finally be observed that the use of fullwave rectifier systems for generating the anode and grid-bias voltages is not essential for the invention although, in view of the smoothing of the rectified alternating voltages, this is to be preferred to the use of half-wave rectifier systems.

What I claim is:

1. A system for supplying anode and grid bias voltages to an electronic amplifier of the class B or C type, said system comprising a first direct voltage supply energized from an alternating voltage source and furnishing anode potential to said amplifier, said amplifier acting as a load on Said first supply, a second voltage supply furnishing constant grid bias potential to said amplifier, means for deriving an auxiliary alternating voltage from said first supply varying in accordance with changes in the load thereon, means to rectify and filter said auxiliary voltage to produce a control voltage whose magnitude depends on the changes in the load imposed on said first supply, and means for combining said control voltage with said constant grid bias potential to produce a resultant grid bias varying inversely as the load.

2. A system for supplying anode and grid bias voltages to an electronic amplifier of the class B or C type, said system comprising a first voltage supply furnishing anode potential to said amplifier, said amplifier acting as a load on said first supply, said first supply including a transformer having a primary winding for connection to an alternating voltage source and a secondary Winding, a rectifier connected to said secondary winding, a filter connected to the output of said rectifier to provide said anode potential and an impedance interposed between said rectifier and said secondary winding, the alternating voltage developed across said impedance varying in accordance with said load, a second voltage supply furnishing constant grid bias potential to said amplifier, means to rectify and filter the voltage across said impedance to produce a control voltage, and means to combine said control voltage with said constant grid bias potential to produce a resultant grid bias varying inversely as the load.

3. A system for supplying anode and grid bias voltages to an electronic amplifier of the class B or C type, said system comprising a first voltage supply furnishing anode potential to said amplifier, said amplifier acting as a load on said first supply, said first supply including a first transformer having a primary winding for connection to an alternating voltage source and a pair of secondary windings, a full wave rectifier tube having a pair of anodes connected respectively to one end of each of said secondary windings and a cathode, a second transformer having a center-tapped primary winding connected between the other ends of the secondary windings of said first transformer and a secondary winding and a filter connected between said cathode and the center-tap of the primary winding of the said second transformer to provide said anode potential, the voltage developed across the secondary winding of said second transformer varying in accordance with said load, a second voltage supply furnishing constant grid bias potential to said amplifier, means to rectify and filter the voltage in the secondary of said second transformer to produce a, control voltage, and means to combine said control voltage with said constant grid bias potential to produce a resultant grid bias varying inversely as the load.

4. A system for supplying anode and grid bias voltages to an electronic amplifier of the class B or C type, said system comprising a first voltage supply furnishing anode potential to said amplifier, said amplifier acting as a load on said first supply, said first supply including a first transformer having a primary winding for connection to an alternating voltage source and a pair of secondary windings, a full wave rectifier tube having a pair of anodes connected respectively to one end of each of said secondary windings and a cathode, a second transformer having a center-tapped primary winding connected between the other ends of the secondary windings of said first transformer and a secondary winding and a filter connected between said cathode and the center-tap of the primary winding of said second transformer to provide said anode potential, the voltage developed across the secondary winding of said second transformer varying in accordance with said load, and a second voltage supply furnishing grid bias potential to said amplifier varying inversely as the load, said second supply including a third transformer having a primary winding connected in parallel with the primary winding of said first transformer and a pair of secondary windings connected in series through the secondary winding of said second transformer, a second full wave rectifier tube having a pair of anodes connected to the free ends of the secondary windings of said third transformer and a cathode and a filter connected between the cathode of the second tube and the center point of the secondary winding of said second transformer to produce the varying grid bias potential.

5. An arrangement as set forth in claim 4 further including phase shifting means interposed between each anode of said second tube and the secondary windings of said third transformer.

PIETER ADRIAAN DONKER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,904,272 Cousins Apr. 18, 1933 1,985,946 Miessner Jan. 1, 1935 2,102,779 Beers Dec. 21, 1937 2,132,830 Norton Oct. 11, 1938 2,154,200 Dow Apr. 11, 1939 

